1. Field of the Invention
The present invention relates to a coating method.
2. Description of Related Art
Coating method of outer surfaces of substrates such as vehicle bodies generally includes steps of coating the substrates and drying coats thereon. The drying step may be divided further into a setting step and a baking step. The setting step is usually carried out in ambient atmosphere or at temperatures of 40.degree. to 60.degree. C. prior to the baking step. The temperature used for the setting step is lower than that for the baking step that is usually carried out at temperatures of approximately 140.degree. C. The setting step may be referred to sometimes as temporary baking.
In many cases coating on substrates may be effected at three steps: undercoating, intercoating and overcoating. Each coating comprises steps of providing a coat on a substrate and drying the coat thereon. An undercoat layer is usually formed on the substrate by means of dipping, while intercoat (intermediate coat) and overcoat (top coat) layers are generally formed by spraying. The paints used for each coating have their own functions: undercoating paints are used to ensure a resistance to corrosion, intercoating paints are to adjust a roughness of the undercoat and to provide an anti-chipping capability, and overcoating is to cover the intercoat. A total thickness of the three coat layers are generally in the range from 85 to 115 .mu.m: a thickness of the undercoat layer being usually in the range from 15 to 25 .mu.m; a thickness of the intercoat layer in the range from 35 to 45 .mu.m; and a thickness of the overcoat layer in the range from 35 to 45 .mu.m.
Overcoating paints are extremely expensive compared with undercoating and intercoating paints so that it is desirable to allow a possibly thinner overcoating while an intercoat layer is rendered as thicker as possible in order to maintain a total thickness of all three coats to conventional thicknesses.
As the overcoat would be made thinner, a color of the intercoat can be seen through the overcoat layer. This may be solved by using the intercoat having a color identical to or very similar to that of the overcoat layer, that is, by effecting the color intercoating.
A limit exists, however, to the thickness of the intercoat layer because a thicker intercoat layer is likely to become roughened as if it is welted to a great extent. Such roughness on the intercoat layer offers the problem with evenness of an overcoat layer formed thereon. This roughness cannot be substantially restored by a wet rubbing treatment that is usually carried out after the drying of the intercoating.
As one standard for evaluating the quality of a coat surface is a degree of evenness. The larger the degree of evenness becomes, the smaller irregularities or roughness on the coat surface. Accordingly, a coat having a larger degree of evenness may be determined as a better coat. It is known that such a degree of evenness on a coat may be improved as the coat is made thicker.
As a paint is coated on the surface of a coating substrate, it may be caused to sag or drop downwardly along the substrate surface due to gravity. The paint may be likely to sag when a large amount of the paint is coated. This sagging phenomenon is a factor of adversely affecting a quality of the coated surface.
The sag may be caused to occur due to the force of gravity so that it may occur on a vertical surface of the substrate. Accordingly, the sagging does not usually offer the problem when a large amount of a paint is coated on a transverse surface of the substrate. This results in the fact that a coat thickness of the transverse substrate surface can be rendered thicker than that of the vertical substrate surface. If a paint would be coated on the transverse surface of the substrate to form a coat as thick as a coat formed on the vertical surface thereof in such an amount as causing no sag on the transverse surface thereof, the paint coated on the transverse surface will be evened due to some extent of its natural flow whereby the coat having a higher degree of evenness is provided on the transverse surface than on the vertical surface.
From the above point of view, conventional spraying procedures have taken the measures to prevent a paint from sagging by using a paint having a viscosity or flowability as low as possible in order to provide a coat surface with a high degree of evenness. For conventional overcoating paints such as thermosetting paints, a sagging threshold value is approximately 40 .mu.m at the maximum. The sagging threshold value is defined herein by the maximum thickness of a coated paint that cause no sags on the vertical surface of a substrate. Sagging of a paint is most likely to occur at the initial stages of the setting and baking steps and in particular at the initial stage of the baking step so that a thickness of a paint to be coated on a vertical substrate surface during the coating step is determined by the thickness of the coat that causes no sagging thereon at this stage, that is, by the sagging threshold value. In order to provide a coat layer with a higher degree of evenness, the conventional techniques require the paint to be coated plural times such as twice by repeating a series of the steps from the coating step to the baking step.
For coating procedures requiring intercoating and overcoating steps, a degree of evenness of an intercoat layer exert a great experience on evenness or flatness on an overcoat layer to be coated thereon. As a limit exists in the conventional procedures, however, to improvement in a degree of evenness on the overcoat layer, a wet rubbing treatment has been carried out over the intercoat layer after the intercoat drying step in order to improve a degree of evenness on the intercoated layer.
The use of the wet rubbing treatment necessarily require additional steps: the step for subjecting the dried intercoat to wet rubbing and the step for drying the wet intercoat after the wet rubbing step. An increase in these steps is disadvantageous from the point of view of commercial production. The wet rubbing may exercise an adverse influence upon a quality of a finish overcoat layer to be coated thereon because portions of a coating surface might be remained wet due to the difficulty of thoroughly drying such a vehicle body as having a complex construction with a variety and number of open portions.